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Message Properties

A message is more than its body

When you publish, the body — the actual bytes — is only half the story. Every AMQP message also carries a set of properties: metadata that RabbitMQ and your consumers can act on without parsing the body. Some are just informational; a couple genuinely change delivery behaviour.

The property that changes behaviour: `delivery_mode`

The one property that affects reliability is delivery_mode (also called “persistent”):

delivery_mode = 2 (persistent) — RabbitMQ writes the message to disk, so it can survive a broker restart.
delivery_mode = 1 (transient, the default in raw AMQP) — the message lives in memory and is lost on restart.

Pair this with a durable queue (last lesson). The rule is worth memorising: durable queue + persistent message = survives a restart. Miss either half and the data is at risk.

Persistence isn’t free — writing to disk costs throughput. That’s a real trade-off you’ll weigh in the reliability module.

The informational properties

The rest describe the message so consumers (and tooling) can handle it correctly:

Property	Purpose
`content_type`	e.g. `application/json` — how to interpret the body
`content_encoding`	e.g. `gzip`
`headers`	Arbitrary key/value metadata (also used by the headers exchange)
`correlation_id`	Match a reply to its request (the RPC pattern)
`reply_to`	The queue a reply should be sent to
`message_id`	A unique id — handy for idempotency and deduplication
`timestamp`	When the message was created
`priority`	Higher-priority messages jump the queue (needs a priority queue)
`expiration`	Per-message TTL in milliseconds

Publishing with properties

channel.publish('orders', 'order.created', Buffer.from(JSON.stringify(order)), {
  persistent: true,          // delivery_mode = 2
  contentType: 'application/json',
  messageId: order.id,
  timestamp: Date.now(),
  headers: { source: 'checkout' },
});

channel.basic_publish(
    exchange="orders",
    routing_key="order.created",
    body=json.dumps(order),
    properties=pika.BasicProperties(
        delivery_mode=2,               # persistent
        content_type="application/json",
        message_id=order["id"],
        headers={"source": "checkout"},
    ),
)

ch.PublishWithContext(ctx, "orders", "order.created", false, false,
    amqp.Publishing{
        DeliveryMode: amqp.Persistent, // delivery_mode = 2
        ContentType:  "application/json",
        MessageId:    order.ID,
        Timestamp:    time.Now(),
        Headers:      amqp.Table{"source": "checkout"},
        Body:         body,
    },
)